Interpretive Summary: The responses of terrestrial ecosystems to climate change are dependent, in no small part, on the availability of soil nutrients plants require for growth and development. This experimental report explains how exposure of a native prairie in south-eastern Wyoming to higher ambient CO2 concentrations (600 compared to today’s approximate 385 parts per million) and warmer temperatures (2/5° F above ambient temperature during the day/night), conditions expected in the second half of this century due to climate change, alter soil nutrients and affect plant responses. Since plants require a balanced supply of soil nutrients to meet their metabolic requirements, an important aspect of this investigation is determining how warming and CO2 affect the balance of two especially critical elements, nitrogen (N) and phosphorus (P). The results indicate that rising levels of atmospheric CO2 tend to reduce the availability of soil N relative to P, while warming has just the opposite effect; it tends to reduce the availability of P relative to N. Plants as well as microbial organisms involved in nutrient cycling appeared to have some flexibility in adjusting their metabolisms to these altered proportions of elements, although the results suggest that N and/or P may both become more limiting to plant growth in the future, depending on the degree to which CO2 increases and warming ensures.

Technical Abstract:
Primary productivity in terrestrial ecosystems is often co-limited by nitrogen (N) and phosphorus (P). While climate change affects terrestrial N cycling with important feedbacks to plant productivity and carbon sequestration, impacts of climate change on the relative availability of N with respect to P remain highly uncertain. Here we show that atmospheric CO2 enrichment (to 600 ppmv) reduced N availability to plants and microbes relative to that of P, and that warming (1.5/3.0 °C above ambient temperature during the day/night) reduced P availability relative to N in a semiarid grassland. The stoichiometric ratio of available N to P was sensitive to soil moisture, suggesting that wetter soil conditions under elevated CO2 and drier conditions with warming were responsible for these relative changes in N and P availability. Plants and soil microbes were highly responsive to alterations in the relative availability of N and P, showing a high degree of flexibility in their biomass N:P ratios. Stoichiometric flexibility may allow plants and microbes to adjust to changes in N and P availability caused by climate change. Based on our observation, warming may alleviate N constraints on plant growth under elevated CO2, while plant growth in a warmer and drier world may become increasingly more constrained by P.